1. Field of the Invention
The present invention relates in general to diverter valves, and more particularly to four way valves associated with bidirectional provers.
2. Description of the Related Art
In typical pipeline systems that transport various liquid hydrocarbon, for example at marine terminals where liquid hydrocarbon is loaded onto vessels as cargo, it is important to measure the flow of hydrocarbons accurately as ownership of the hydrocarbon commodity changes and a dollar value is assigned to the volume of hydrocarbon. The accuracy of the flow meter used to measure the volume of the hydrocarbon, such as turbine meter, positive displacement meter, liquid ultrasonic meter, or other known metering device, can be affected by a variation in characteristic of fluid to be measured, changes in operating process conditions, life cycle of flow meter, and other external conditions. A meter prover can be used to check the accuracy of the flow meter by deriving a meter factor. When the meter proving system is being operated, the entire flow of fluids from the custody transfer flow meter is diverted through the prover.
Meter proving systems in the U.S. and internationally, in general, can be guided by The American Petroleum Institute Manual of Petroleum Measurement Standards, Chapter 4-Proving Systems, Section 2, Displacement Provers, Third Edition, September 2003, Reaffirmed—March 2011 (hereafter this standard shall be referred as API MPMS Chap 4.2). Meter proving using a bidirectional sphere prover is approved as per this API Standard.
A typical bidirectional sphere prover can have a length of pipe through which the sphere travels back and forth, actuating a detector at each end of the calibrated section. The flow and prover computer activates totalizing flow measured by flow meter when the sphere passes through detecting points at one end of the calibration section and stops totalizing when sphere reaches other ends of the calibration section. This is the totalized flow meter reading for the time required for the sphere to travel between the detecting points. This reading is then compared with the known volume of the metering pipe between the detecting points to provide an accurate calibration data and meter factor.
A four way diverter valve can be used to reverse the direction of the flow through the bi-directional prover. During proving all the fluid from the flow meter should pass through the prover. However, this should be the only fluid to pass through the prover. No fluid should leak out or creep from in between the flow meter and prover. Leaking of fluids from the four way diverter valve will result in fluid creeping out to the outlet port before passing though the prover barrel, resulting in inaccurate proving and thus inaccurate flow measurement. In the event that the diverter valve leaks immediately before or during a proving operation cycle, the meter factor obtained during proving would be inaccurate. The magnitude of the inaccuracy depends on the extent to which the diverter valve is leaking A four way diverter valve tends to leak when the sealing surfaces become excessively worn through long usage or because of the presence of abrasive materials in the fluid.
API MPMS Chap 4.2 requires that the diverter valve must be leak-free and a method of checking for seal leakage during a proving pass is required to be provided. However specific method of leakage is not specified. Some commercially available methods for checking for seal leakage, such as using a pressure gauge or differential pressure switch, provide reactive indication after failure of seal unit but do not aide in predicting current status on seal integrity in a four way diverter valve. As an example, a differential pressure switch generally provides discrete signal (such as an ON/OFF contact) for remote indication of seal failure, once the seal has failed. A differential pressure switch of some current systems do not therefore provide an indication on deteriorating condition of the seal as early warning so that an operator could take possible preventive actions.
The constant use of proving systems lead to significant wear of seals in four way diverter valves. The replacement of seal units can take a number of weeks. If such provers are used at marine terminals, as an example, ship cargo cannot be proved, which can result in monthly financial exposure in the range of millions of dollars.